2B - Exchange across cell membranes (diffusion)

Question 1

What is diffusion?

Answer 1

The net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower until evenly distributed.

Question 2

Is diffusion active or passive?

Answer 2

Passive

Question 3

What type of molecules move by diffusion?

Answer 3

Only small, non-polar molecules such as oxygen and carbon dioxide.

Question 4

What does facilitated diffusion use?

Answer 4

Carrier and channel proteins.

Question 5

When does facilitated diffusion occur?

Answer 5

When molecules are too big to move by simple diffusion/the process of diffusion is too slow. Also, if the molecules are polar.

Question 6

What type of substances move by facilitated diffusion?

Answer 6

Larger molecules such as glucose and amino acids.

Question 7

Why do charged particles have to move by facilitated diffusion rather than simple diffusion?

Answer 7

They would diffuse slowly because they are water soluble and the centre of the bilayer is hydrophobic.

Question 8

Is facilitated diffusion active or passive?

Answer 8

Passive.

Question 9

What does facilitated diffusion rely on?

Answer 9

Kinetic energy of diffusing molecules.

Question 10

Does simple diffusion and facilitated diffusion occur up or down a concentration gradient?

Answer 10

Down

Question 11

Describe how channel proteins work in facilitated diffusion

Answer 11

Form pores in the membrane (water filled hydrophilic channels) which allow water soluble ions to pass through.

They are selective as different channel proteins facilitate the diffusion of different charged particles.

The channel will remain closed unless specific ion is present.

Question 12

Describe how carrier proteins work in facilitated diffusion

Answer 12

A specific large molecule attaches (binds) to a carrier protein in the membrane.

Then, the protein changes shape as a result.

This releases the molecule on the opposite side of the membrane.

Question 13

Which protein helps with the diffusion of charged particles across the cell membrane?

Answer 13

Channel proteins

Question 14

Which protein helps with the diffusion of large molecules across the cell membrane?

Answer 14

Carrier proteins

Question 15

What are internal cell membranes?

Answer 15

The membranes that are surrounding organelles such as the mitochondria.

Question 16

What does the rate of simple diffusion depend on?

Answer 16

The concentration gradient - the higher the conc. gradient, the faster the rate of diffusion. The difference in conc. between the 2 sides of the membrane decreases as equilibrium is reached so diffusion slows down over time.

The thickness of the exchange surface - the thinner it is, the faster the rate of diffusion as the shorter the distance the particles have to travel.

The surface area - the larger the SA, the faster the rate of diffusion.

Question 17

Explain how one feature in the body is adapted to increase the rate of simple diffusion

Answer 17

Microvilli increase the SA of the epithelial cells in the small intestine for a faster rate of diffusion.

Hair-like projections.
Increase the SA around 600x.
Therefore more particles can be exchanged in the same amount of time - increasing the rate of diffusion.

Question 18

What does the rate of facilitated diffusion depend on?

Answer 18

The concentration gradient - the higher the conc. gradient, the faster the rate of facilitated diffusion up to a point. As equilibrium is reached, the rate will level off.

The number of channel or carrier proteins - once all the proteins in a membrane are in use, facilitated diffusion can’t happen any faster even if you increase the conc. gradient. So the greater the number of channel or carrier proteins in the cell membrane, the faster the rate of facilitated diffusion.

Question 19

Explain how one feature in the body is adapted to increase the rate of facilitated diffusion

Answer 19

Having more special channel proteins called aquaporins in the kidney cells allows the diffusion of water through cell membranes.

They allow the cell to reabsorb a lot of the water that would otherwise be excreted by the body - about 180 litres need reabsorbing every day.